1. Field of the Invention
The present invention relates to an optical displacement measuring apparatus, and more particularly to an optical displacement measuring apparatus applicable to encoders for measuring a displacement or a velocity of an object, velocity sensors, acceleration sensors, length measuring apparatus, etc., utilizing the fact that when light impinging on a moving object is diffracted and scattered, the thus diffracted and scattered light is subject to phase modulation according to a displacement or a moving velocity of the object.
2. Related Background Art
As conventional apparatus for obtaining an amount of movement or displacement of an object at high accuracy there have been used, for example, optical encoders, laser Doppler velocimeters, laser interferometers, etc., utilizing interference of diffracted or scattered light beams from the object illuminated with light.
A specific feature of these apparatus utilizing the light is the feasibility of easily achieving high accuracy and high resolution of the wavelength order of light, but there is a demand for miniaturization (into the size of the millimeter order), to enhance the stability, "easiness to handle," and durability of the interference optical system, etc. in order to be applied in further wider fields.
The applicant proposed in the bulletin of Japanese Laid-open Patent Application No. 5-340719 a high-accuracy optical linear encoder utilizing interference based on diffracted light phase-modulated by a moving object. The same bulletin discloses an optical encoder that is simple in arrangement of the optical system, stable in interference between diffracted light, easy in handling, and suitable for miniaturization.
FIGS. 1 to 3 are schematic diagrams to show the major part of the optical encoder proposed in the same bulletin. Among them, FIG. 1 is a perspective view of the major part, and FIGS. 2A and 2B are schematic diagrams of the major part. In FIG. 1, 1a designates a light-emitting element such as a laser diode, CL a cylindrical lens, and ASP an aspherical lens, on a flat surface side of which a beam splitting diffraction grating G1, as detailed below, is disposed. The cylindrical lens CL and aspherical lens ASP compose an anamorphic optical system. Further, 4a denotes a transparent substrate, on which a beam superimposing diffraction grating G3 as described below is disposed. Reference numeral 20 represents a scale attached to a detected object relatively moving, on which a diffraction grating G2, as detailed below is disposed. Here, G1 is the beam splitting diffraction grating (for example of grating pitch P1=1.6 .mu.m), G2 the diffraction grating (for example of grating pitch P2=1.6 .mu.m) disposed on the scale 20, and G3 the beam superimposing diffraction grating (for example of grating pitch P3=1.6 .mu.m).
The beam superimposing diffraction grating G3 is placed on the same plane as the beam splitting diffraction grating G1. The beam superimposing diffracting grating G3 includes four parts G3a, G3b, G3c, G3d, which are formed at grating positions shifted from each other as shown in FIG. 3, thereby providing light beams incident to the respective parts with relative phase deviation .pi./2 between them. PD denotes a quartered photoelectric element (light-receiving element) composed of photocells PDa-PDd.
As constructed in the above structure, the optical displacement measuring apparatus operates as follows. A diverging light beam R emitted from the light-emitting element la is converted into a linearly converging light beam R' by the cylindrical lens CL. The beam next enters the aspherical lens ASP, and thereby the beam R' becomes emergent from the rear plane of the aspherical lens ASP so as to be condensed near the diffraction grating G2 in the direction of grating lines and nearly is collimated with the direction along the arrangement of grating lines. On this occasion the light is split by the beam splitting diffraction grating G1 into two light beams, i.e., zeroth-order diffracted light beam R0 advancing straight and +first-order diffracted light beam R+1, which are incident on the scale 20 relatively moving. In this event, the center rays of the respective light beams are incident at points P1, P2 on the diffraction grating G2 on the scale. Among the light beam R0 linearly illuminating the point P1, a light beam R0+1+first-order-diffracted by the diffraction grating G2 is diffracted by the diffraction grating G3. Among diffracted beams a light beam R0+1-1-first-order-diffracted there is emergent nearly normally from the diffraction grating G3.
Among the light beam R+1, a light beam R+1-1-first-order-reflection-diffracted by the diffraction grating G2 is incident to the diffraction grating G3. Among those diffracted a light beam R+1-10 traveling straight through the diffraction grating G3 is emergent nearly normally from the diffraction grating G3. The two beams R0+1-1, R+1-10 are superimposed on the beam superimposing diffraction grating G3 to interfere with each other with their wavefronts overlapping, then entering the quartered photoelectric device PD.
Employment of such an arrangement of the optical system enables the encoder to achieve high accuracy and being easy to handle, because the interference state is stable even with the occurrence of relative positional deviation (angular deviation such as azimuth, tilt, etc.) between the "diffraction grating G2 on the scale" and the "detecting head portion consisting of the beam splitting diffraction grating G1, beam superimposing diffraction grating G3, light-emitting element, photoelectric element, etc."